When he isn't saving the world, Thomas Cahill writes stories about the end of the world.

Over several decades, Cahill's scientific work -- he is now professor emeritus in the UC Davis Physics Department -- has greatly effected the lives of regular people. His 1973 research on toxic airborne lead resulted in the adoption of the catalytic converter in all automobiles. More recently, he has investigated global climate change for the National Oceanic and Atmospheric Administration, and was first to warn of airborne health risks following the collapse of the World Trade Center in New York.

Cahill was scheduled to speak at 7:30 p.m. Saturday, at The Avid Reader, 617 2nd St., Davis, about his newest book "Ark: Asteroid Impact," but a medical emergency has forced him to delay the gathering, which will be rescheduled in the future.

During his down time, the scientist likes to relax by writing science fiction novels.

"I grew up in New England without television, living on science fiction -- Arthur C. Clarke and Asimov," Cahill said. That gave him a lifelong love for the genre, and "I always felt like that was something I'd come back to later in life."

He has, with a vengeance. Cahill's latest novel, "Ark: Asteroid Impact," is the first in a trilogy exploring a terrible scenario: A giant asteroid strikes the Earth, destroying billions of lives and civilization as we know it, and forcing small groups of survivors -- including the book's hero, a UC Davis physicist -- to get by in a post-apocalyptic wasteland.

"Ark," published by Davis-based EditPros, follows closely behind "Annals of the Omega Project," a three-part thriller delving into telepathy, of all things. This Saturday, the writer will be appearing at The Avid Reader bookstore in Davis to discuss his work -- and to alleviate any fears over DA14, another asteroid due to breeze by the planet on Feb. 15.

Naturally, Cahill's stories are infused with real science, making for an informative read. The "Ark" trilogy draws heavily on his expertise in atmospheric science, leading to lines such as, "There is a hint of yellow in the sky -- most likely the precursor of the sulfuric acid cloud."

Even the specific asteroid that seals our doom is based in science: 4179 Toutatis, a 3-mile-long, barbell-shaped rock the size of a mountain, approaches Earth every four years, with its latest fly-by occurring less than a month ago.

Scientists believe Toutatis won't come scary-close for another 600 years or so. So for the purposes of "Ark," Cahill had another, unknown asteroid modify its course ever so slightly.

"It's a biggie. So when it hits the Earth, things get really bad in a hurry," he said.

From there, the story taps into "my own knowledge of global climate," he said. And Cahill believes the impact would result in a cold snap that is "pretty severe but pretty short. So I tried to put as much science as possible into it."

Cahill said his background is a double-edge sword, explaining, "The science I think helps me because it makes things more realistic, but hurts me because it does constrain things I would otherwise do."

"I'm not a great writer, let's face it," he deadpanned. "I'm not into teenage angst."

One could be excused a little angst over rocks like DA14, which on Feb. 15 will brush so close to Earth that "it's coming inside the orbits of the communications satellites," Cahill said.

According to NASA, this asteroid is only about 45 meters across, meaning its impact with Earth would "only" result in a 2.5 megaton explosion.

And in any case, it's not expected to hit the planet. But "it's eventually likely" that the planet will be hit again by a big rock, Cahill said.

"The Earth has been hit with a lot of very large asteroids in the past, and nobody ever realized it."

And when that happens, "What would you do?" he asked. "Where would you go? How would you survive?"

The questions are enjoyable distractions for a scientist otherwise working on everyday problems.

"The big deal right now is we're trying to find a way of protecting people from the effluence of freeways," he said. In particular, the use of brake drums and pads is resulting in very fine particles of heavy metals lodging in the lungs of people living or working near high-traffic zones -- including kids attending schools near freeways.

So far, he said, researchers have determined that "regulations of the U.S. are inadequate to protect people living near highways."